Reenforced-concrete paving



l 1,623,090 P" 1927' s. o. BURR REENFORCED CONCRETE PAVING Filed Aug-31, 1925 4 Sheets-Sheet. 1

INVENTOR I George D. Burr I 1,623 090 I Aprfl 5, 1927. G. D. R

REENFORCED CONCRETE PAVING File 31. 1925 4 Sheets-Sheet 2 INVENTORGcaorge D.;Burr

ATTORNEYS G. D. BURR REENFORCED CONCRETE PAVING.

Filed Aug. 31. 1925 4 Sheets-Sheet 3 INVENTOR George D. B

TORNEYS I A ril 5 192 p 7 I s. D. BURR REENFORCED CONCRETE PAVING 4Sheets-Sheet 4.

Fil ed Aug. 31, 1925 INVENTOR George D.Burr

9 ATT NEYS Patented Apr. 5, 1927.

PATENT OFFICE.

GEORGE D. BURR, 0F SEATTLE, WASHINGTON.

REENFORCED-CONCRETE PAVING.

Application filed August 31, 1925.

My invention relates to paving, and more especially to paving ofconcrete, laid in place preferably in slabs upon a prepared subgrade,and generally requiring the employment of reenforcing material imbeddedin the concrete. My invention relates, further, to the method of pavingby means of which the necessity for the employment of reenforcingmaterial, the kind of reenforcing material, and its location in theslab, may be determined in accordance with sub-grade conditions andtraflic to be expected, whereby the concrete may be laid in such mannerand in such forms, and with the reenforcing materials (if required oremployed) so disposed therein as to effect the most economical use ofthe concrete and reenforeing material, based on sub-grade and trafficconditions, and resulting in the production of a pavement whichthroughout its length is uniform in its bearing power, notwithstandingvarying conditions of sub-grade and traffic to be expected.

Considered chiefly as applied to reenforced concrete paving (althoughpavement without reenforcement may be laid in accordance with theprinciples of this invention, as will be apparent as thedescriptionprogresses), my invention relates to such paving and to the process oflaying the same which consists in laying the concrete and disposing thereenforcement therein in direct relation to the observed or calculatedbearing power of the sub-grade at any particular point. as will be morefully apparent from a study of the following specification, includingthe drawings, and of the appended claims.

The laying of concrete paving, to resist the stresses it is designed towithstand, has been largely a matter of guesswork. It has been laid inthick slabs, often of uniform thickness through its width and length,such as were considered sufficient to withstand the maximum stresseswithout breaking down, and without reenforcement. No account was takenof the bearing power of the sub-grade upon which the concrete was laid,and which in a pavement of any considerable length is bound to vary,with the result that for sub-grades of high bearing power, the excessthickness of the concrete (and often the Serial No. 53,542.

preparation of such a hard sub-grade for the reception of the concrete)entailed considerable wastage, even though the thickness might bedecreased for sub-grades of known high bearing power, if laid of uniformthickness through its width the excess thickness at points in the widthwhere the stresses are a minimum, entailed a wastage.

To avoid wastage of the class mentioned, it has been proposed toincorporate reenforcing matter in the concrete. Through lack ofknowledge of the bearing power of the sub-grade beneath the severalportions of the paving, and further by reason of the necessity of makingthe paving as nearly as possible uniform, due to the unskilled laboremployed in laying it, the use of reenforcement in concrete paving hasbeen done in much the same manner as the laying of such pavement withoutreenforcement. that is, without regard to the bearing power of thesub-grade or the value of stresses in the several portions of the pavingin determining the sizes or location of the reenforcing materials. Steelis recognized as being of far greater strength in tension than concrcte,and hence where tension is found it is desirable to use steel. However,the steel which has been employed has been put in largely as a matter ofguesswork; if steel of a given concentration and strength, placed at acertam point did not cure defects in the roadway, it was tried at otherpoints, or

with different arrangements or concentrations. It was used, thus,uneconomically, primarily because of lack of knowledge of a bettermethod or system of obtaining the desired results, nan'iely, a pavingsurface sufficiently strong to carry the traffic loads. Generally thereenforcing materials have been equally spaced throughout the pavingslabs, both laterally and longitudinally, and often of uniform strengththroughout. )Vhile the use of reenforcement may avoid wastage ofconcrete, its indiscriminate use, as heretofore employed, entails anobvious wastage of the reenforcement.

Data relative to stresses applied to an existing in concrete roads withrespect to traffic and with respect to the effect of a given sub-grade,has been made available through publication from time to time of theresults of tests conducted by the United States Bureau of Public Roads,at Arlington, Virginia, and by the State of Illinois on the Bates road.Later data from the same source, and tests conducted by theCalifornia'State Highway Commission at Pitts burgh, California, givesthe distribution of the load on the subsgrade with the central load onthe slab. Reference to all of these tests will show, however, that theyhave done no more than to sjive the re ults of certain specificexperiments as to the results obtained by tests under certain specificconditions and. on certain specific pavings. They enable the engineer'tooetermine by experiment on any special paving after it is laid,ofyarying sub-grade and trattic conditions, and they point out to himthe result of varying sub-grades and tralhe conditions upon got-hespee-i'ficpaving tested, but they do not point to any system whereby theengineer an calculate beforehand what will be the efleetiiupon thepavin; le proposes to l v. vof the existing sub-grade and expectedtraftic conditions, nor do they enable him to see when reent'orcing isrequired. nor of what character and-strength, or where it is to be,placed in the slab. These tests will indicate to the engineer,generally,when reenforcing of some. kind required, but he is stillconfronted'with the problem of designing his paven1ent to care for thesub-e ade conditions which may exist at any particular "point -in hispaving, and l of determining "where' the 'reenforcement is to he placedin the slab, and of what strengthandcharacter itlisto be.

My methodis concerned with the determination ofthe character of paving;to be 40 employed, and the character and positioning of thereenforcement to he required, if any, iin the paving slabs, startingwith the results :ofthe experiments mentioned, as a basis. Accordingtomy method, therefore. I e l ploy reenforcing material at such points andinsueh manner and of such strength as to resist stresses which can beforeseen through observation of the bearing; power of the sul "grade andthrough calculation of the probable stresses clue to the passage ofloads over 'the pavement, eliminating the reentorcingg; materiahwherethestresses to be expected and thereaetion of the sub-grade, render it:unnecessary, and I so form the concrete slabs as'to most effectivelyresist the same .stresses,'thus effecting an economy ofconcretematerials, reenforeing materials, and labor, and producing acheaper and better pavement without requiring; the use of more skilledlabor in the laying; of the p: ving.

My invention will be best understood by reference to the accon'ipanyingdrmving's, whereinI have illustrated my method of fpavin-g andreenforcenient theretor. con- 'structed in furtherance of my method andaccording to my invention, in which Figure l is a diagram illustrating:a application of the method for determining the reenforcement to beeu'iployed.

Figure 2 i-' a perspective view enforcing: tor a single paving slab,extending over the full width of the road.

Figures 3 and tare sections lougitinlinally of the slab on therespective lines 3--3 and -ill of Figure 2.

Figures and (l are sections transversely of the slab along therespective lines 5-5 and (l6 of Figure 2,

Figure 7- 'is a plan view ofa slab designed for a road with a centerjoint.

Figures 8 and 9 are sections lon g itudinall of the road on therespective lines 88 and 99 oflligure T, and l? igures 101 and ll aresections transversely of the slab, along the respective lines 10 10 and11l11 of Figure7.

Certainassrunptions are nece sary and can be accepted as provenexperience, and. by the results of the tests mentioned above. as a basisfor my n'iethod of design. Thes are as follows: (1), that the sulrgradeis uniformly ela tic up to the limit of the hearing power use l. (2),that the sub-grade bearing power is as determined by using a bearingblock at least live square feet in area, and is the reaction obtainedwhen the deflection of the sub-grade is 0.12 inch under the point ofapplication of the load, (3), that the elastic curve of theloaded slabis shaped as (lo-- termined by the Arlington experiments, (4), that halfof the load is carried by the sub- 'rade between the points ofcontraflexure of the elastic curve when the slab is loaded at the edge,(5), that when there is a sudden changre from a hot to a coldatnmsphere,tlle corners of the slab curl up so as to receive no support from thesub-grade at all. llhese assumptions are all well founded on experience,as has been noted.

There is one more assumption which. may be said to be till in a slightlyexperimental stage, and eh can be. alled unption (6). it is that whenthe central portion of the slab is designed with. a thickness c0rre tothat generally found necessary in accorc t with the Arlingtonexperiments, all lures will be at the corners or will start at theedges. The result of this assumption is that reenforcing bars-in themiddle are only required for the purpose of 'buting stresses, as, forexample, tempeiature stresses .and taking care of ients caused byunintentional local setof the sub-grade. Further reeni'orcel'nent in thecenter of the slab, is nonnor is it even (lt I able, contrary l raccepted practice. has been employed in designinn thereeulorcenientplaced in a concrete road in King County, Vashington,known spondin of the rellli) llll as the Kenton-Maple Valley road,wherein stretches rendered diitticult by the character of the subgradewere selected to test the soundness of my method of design and the valueof the reinforcement. This road has now been laid and in use some tenmonths, and no failure is observed. It was laid on Fresh fills and wheredrainage conditions and subgrade conditions were particularly badbecause of the presence of large quantities of water and a particularlyboggy subsoil. For the purpose of illustrating my method I will give thefigures employed there, it being understood of course that these aregiven for purposes of illustration only, and it is not to be understoodthat these precise figures are in any sense restrictive. They are givensolely for the purpose of illustrating, by reference to an actualconstruction, how my method is carried out and the results in the formof the reent'orcing steel necessary, and its positioning.

On the reentorced sections of this Benton- Maple Valley road theprocedure was as follows. The sub-grade, after being prepared. wasmeasured for its bearing power. This was done by placing upon it a slabof not less than five square feet area, and loading this slab until thedeflection of the sub grade was 0.12 inch. This can be done by suchinstruments as are found suitable and Figure 1 illustrates an elementaryform of apparatus which might be employed for the purpose. Therein aloaded truck is shown upon the roadway, but partially supported by ahydraulic jack J controlled by a pump P, having a gage G by means ofwhich pump a load (the truck) can be applied to the sub-grade throughthe five square foot slab S and jack J, resting thereupon. The load onthe sub-grade through the slab S is increased until the deflection ofthe subgrade is 0.12 inch. The deflection can be observed and measuredthrough a scale or scales A, observed through a. theodolite T, or likeinstrument. If two squares A are employed they will compensate forirregularities in the surface or for local variations in the bearingpower of the sub-grade or for compression of the materials of the jackitself or the slab S. The observation through a theodolite from adistance will likewise prevent local settling of the sub-grade or likedisturbances caused by the placing of the load on the jack, fromdisturl'iing the accuracy of the reading.

At one point in the Renton-lvlaple Valley road, referred to. the bearingpower as thus measured, for a. 0.12 inch deflection of such a fivesquare foot slab, amounted to 000 lbs. per square foot. The axle loadfor which the road was designed amount to 18,500 lbs. plus fifty percent for impact. The strength of the concrete in compression was takenat 800 lbs. per square inch, and the strength of the steel, in tension,at 24,000 lbs. per square inch. Employing the chart for the requiredthickness for centers, from impact tests at r\rlington, with the wheelload of 0250 lbs., I found a six and one-halt inch slab was suflicientfor the central portion of the road. The edges were thickened to nineinches. .By measurement it was found to be general for six and one-halfinch concrete slab, where there is 0.12 inch deflection under the load,there is an 0.10 inch deflection at the point otcontrafiexure of theelastic curve. This fact might be calculated, but calculations of thistype are long and laborious and have never yet been accomplished and theresult is more easily obtained, well within the limits of accuracyrequired, by means of actually measuring the elastic curves. asmentioned above. Thus if a load is such as to deflect the slab 0.12inches under the point of application of the load at the point ofcontraflexure of the elastic curve the deflection can be assumed asexactly 0.10 inch. Ten-twelfths of 900 lbs. equals 7 50 lbs. which isthe sub-grade bearing at the point of contraflexure (see assumption 1).

2 =825 lbs.

A 2/3 Lg, and L HE;

but

1/2(load 50% impact) average sub-grade bearing, therefore 6937.5(3) V IJ11101163,

approximately. The maximum moment M will then be as for a simple beamwith span L and concentrated load of 6937.5 lbs.

The longitudinal edge of the slab is now 60 104,000 inch lbs.

designed as a simple reen'torced concrete beam. This is a simple andasomewhat approximate way of obtaining theresult, but the whole purposeof the present method is to obtain a system simple enough and accurateenough for general use.

It will be seen that the higher the bearing power of sub-grade theshorter the distance between points of cont-ra-flexure and conse quentlythe smaller the amount of steel re quired, until we reach the sub-gradethat 17equires no steel at all, in the bottom 01"" the edges of theslab. Reversely, the softer the sub-grade the stiller must the pavingslab be made by means of reen'torcement, in ac cordance with thedefinite law outlined above. Negative bendingmoments beyond the pointofcontraflexure are small in comparison to the positive moments, andusually not in excess of the tensile strength of the concrete. However,to take care of these negative moments in case they should becomeexcessive to any reason'by'local settlement oi the slab, a inch bar isrun along the top of the edge. This will be seen at 1 in Figures 2 andl. This bar 1 also serves; to resist stresses which arise throughchanges in ten'ipcrature or shrinkage, and which would produce tensionalstresses throughout the top of the edge of the slab.

The principal longitudinal bars, it will be seen, are chieflyrequired-atthe bottom of the side edges, intermediate the ends of theslab. These are indicated at 2-and 20. The bending moments at thecorners, however, will largely be negative, and the ends of these barswould have little effect in resisting compressional stresses thusoccurring. However, the ends of the bars must be anchored in theconcrete for sufficient bond to enable their central portions to acteffectively, and further, the corners are acted upon by moments whichproduce tensional stresses in their upper portions. Therefore, bybending the ends of the bars upward as shown at 21 and 22, such negativebending n'ioments, occasioned as outlined in assumption 5, are takencare or bythe ends, and econon'iy of material is'e'tl'ected. Further,

it is possible to stiffen this element of the reentorcing struetu e, andconsequently the entire-structin'e, by tying certain of these bent-upends'to the ends otthe bars 1, as indicated at 12. The maximum moment atthe corners is a function of the sub-grade and the relative temperatureand moisture content of the top and bottom of the slab, but for mostpractical work with the thicln ness of slabs usually employed, it may beconsidered being at a point two to three and one-half feet distant fromthe corners. It is here that I have found. from experience slabs usuallyail.

It may also be desirable to assist in -aring tori cal stresses to extenda bar, as 3,1ne1lt5 through the edges of the slabtron'i end to end atits bottom. The position of the bars 2, 20,11 and 3, that is, as towhich outermost, isto a large extent immaterial, but the" should all befairly close to the edge. For best results, I prefer to space the bars 1and 2 substantially four inches inward oi the edge of? the slab, the bar3 eight inches inward of the edge of the slab,.aml the bar 20 twelveinches inward ot' the edge, the total width ot the road *ay being twenty:teet. The ob ect is to place the reeni'orcing as close as practicaaleto the edge. It the length ot slab in such a roadway is twenty teet, thesections 21 and 22 may be four "feet three inches in length, and thebent sections 2?, and 2t, respectively, may extend through a 'l'oot inlength, making the sections and 20 nine feet lon Lam '(LZ9"001'170100112021 t.

This theory ot lateral ticxure of paving slabs shows. that maxinn-unmoments, except at corners, occur at the middle of the ends of the slab,and are both positive and negative. The greatest otthese are negative,due to the curling up ot thoedgcs oi the sh b. the softening or thesub-grade near the edges of. the pavement by saturation withwater, andby the action-0t :lrost in the center. In the lateral reentorcement l.have neglected impact.

vFor such a twenty-toot pavement as used for illustration. the maximumclear distance apart between the two lines of traliic is considered-t-obe seven feet. The. total of the two axial loads in line is 37,000 lbs,oi which threedourths is transmitted to the sub-grade outside oi theseven-toot clear distance. The remaining one-fourth, 010250 lbs, can beconsidered unitormly distributed on the subgrade along the s-eve1rfootinterval. The resulting moment. is

='97,125 inch pounds.

The top reentorcement of the edge is ccordingly designed under themethoddescribed in connection. with the longitudinal edge, that is,'as-a beam.Jlt will be noted that the narrower the pavement the smaller .the amountof top lateral reenlorcement required. The bottom lateral reentorcementis designed in exactly the same manner as the longitudinalreentorcen'ient, except that the allowance of 50% for impact has beenomitted.

I find thus that at the transverse edges of the slab, bars l, 4:0, 41and 42 are required, extending. across the slab adjacent its edges andin the upper portion of the slab. Like transverse bars. 5 and. 50 areemployed in the bottom of the slab, the immh-er being less. because ofthe lower value ot the positive moas compared with the negative mofltlments. The bars 4, 40, 41 and 4:2, as will be seen in Figure 6, aresubstantially straight, at least may be left unbent and will conform tothe crown of the road as they are connected to the bars 1, 2 and 20. Thebars 5 and 50, however, are preferably bent as indicated in Figure 6, toconform to the thickening of the edges of the slab.

To distribute local stresses I prefer to incorporate in the slab,longitudinal bars 6, 60 and 61, placed generally in the bottom of theslab, in accordance with the principles involved in the employment ofthe bars 2 and 20, although the bar 6 is placed in the center at the topand other like bars may be employed if their use is found desirable.Thus the center of the slab, with the excep tion of the bars 6, 60, and61, and with the exception of the transverse bars 7, which are likewisein the bottom of the slab, is free of reenforcement. The bars 6, 60 and61 and the bars 7, are ordinarily of lighter material than the barsplaced at the edges, .and heretofore described, their chief functionbeing, as stated, to distribute local stresses and not to resiststresses due to expected sub-grade conditions. The main reenforcing bars1, 2, 3, 4C and 5, are altogether in or near the edge of the slab, wherefrom experience and by reason of the method outlined above, it appearsthat theyare most required.

Of course it will be understood that the bars may be secured together inany way found suitable or which may be desired, and that spacing meanssuch as the stirrups 8 may be employed to maintain them in properposition until imbedded in concrete. The precise manner of securing thebars together or of spacing them forms no part of my invention.

The same principles may be employed in designing a slab having a centerjoint, as shown in Figures 7 to 11, inclusive. This differs from theformer design chiefly in the inclusion of the bars 2 at the centerjoint, that is, imbedded in the slab just Within the center jointindicated at 9. This is required because of the use of two adjoiningslabs so that the design of the central portion, that is, parallel tothe center joint, must be in accordance with the principles outlined forthe design of the edge of the slab. I prefer, however, that thetransverse bars 5 and 50, 4-, 41, 42, etc., extend completely throughthe two half slabs in this form, although each half slab may be formedindependently or semi-independently of the other, if desired.

It will not be apparent that I have evolved a method of laying suchpavement in ac cordance with the bearing power of the subgrade in anyparticular location. The method, briefly, consists in the determinationof the bearing power of the sub-grade,

originally through the use of apparatus similar to that illustrated inFigure 1, but frequently merely as the result of observation andexperience, and then the application of the data thus obtained, togetherwith the known strength of the materials to be employed, and the loadsto be expected, in simple formuli, by means of which the nature andlocation of the stresses in the concrete may be determined. lVith thisknown, and following the principles outlined herein of treating theedges of the concrete slab as loaded beams, it is a simple engineeringfeat to determine the strength and number of bars to be employed. Barsof the proper strength and sufiicient in number, as calculated by theengineer in charge, are then prepared in forms and tied together as inthe illustration of Figure 2, and laid in place on the preparedsub-grade. If in a location where the bearing power is high, certain ofthe bars, as indicated by the application of the formula and theprinciples outlined above, as, for example,

the bar 3 or the bar 20, may be eliminated. The concrete, of course, ispoured about, beneath, and over the reenforcing material, this beingsuitably supported by any of the well-known means during the process ofpouring the concrete.

The method thus outlined and the reenforcementthus designed, providesthe maximum resistance to the various stresses occurring in the concreteslab, with the minimum of material, and as noted, provides a meanswhereby the amount of reenforcemcnt may be calculated and may be variedto suit the conditions at any parvticular point in the paving.

It will be noted that the joints between bars in the reenforcingstructure are tied together, and consequently the entire structure maybe set up through the use of proper template forms, either on a portionof the completed paving, or at the side, and then carried as a unit tobe laid in place on the prepared sub-grade for the next section to belaid. In this manner there is no delay, there is no tying of the steelbars in place, and the mixer and its crew are never forced to remainidle to enable the positioning or tying of the reenforcing structure.

What 1 claim as my invention is:

1. A paving slab comprising a monolithic concrete structure including aplurality of reenforcing bars extending from end to end of the slab atits side edges, certain of said bars lying in the lower portion of theslab edge intermediate its ends, and the ends of said bars lying in theupper portion of the slab edge, the remainder of said bars lying whollyin the upper portion of the slab edge, the ends of said latter barsbeing secured to the ends of the first bars to form a truss-likestructurelyin'g; inthe side edges of the slab,

and saidslab inward of its edges, being-'1 largely free of longitudinalreinforcement.

2.- Reenforcing:lfor concrete paving slabs comprisinglongitudinal barsadapted in the ina inito lieeinthe-lower portion ofthe side cd esofthecompleted slab, the endsof'said' bars beingbent upwardly and:extending longitudinally to :the ends of the-slab, andadapted-to lie inthe upper portion of the slab edges,- other longitudin al barsadapted tolie whol-lyin the upperportion of the slab edges;and:transverse barsadapted to lie-intheenctedges'and extending. from-side to side- 0f theslab.

3. Reenforcing for concrete pavingslabs comprisinglongitudinal barsadapted in the main to liein-the lower portion of the side edgeso-fithecompleted slab, the ends of saidba-rs being bent upwardly and thence outwa-rdly to lie inthe upper portion of the slab side edges-, otherlongitudinal bars adapted to lie Wholly in the upper portion ot the slabedges, and transverse barsada-ptedto lie in the end edges ofthe slabwhen completech certain-ofsaid transverse bars lying'in the upperportion ot the end edges and the remainderlying in the lower portionthereof.

4. A: reenforcing structure for concrete )aving slabs comprisinglongitudinal bars adaptedlin the main-to lie in the lower portion oftlre side edges of the completed isla'b, the endsof said bars-beingbentupwardly and then extending-.longitudinally to :lie in the upper:portion of the slab side: edges, other longitudinal bars extending'flOll] end to 'end 5' ofthe sideedges at top and 'bottom',respectively, and transverse bars extending from side to side-atithe endedges, and 'positioned at the top and bottom, respectively, stirrups'connectingtheupper and lower transverse: bars, said bent longitudinalbars being-connectedto the upper and lower l0n-' gitudinal :bars, andsaid transverse and :lon-:

gitudinal bars being secured together to form a complete,portablestructure;

5. A- reenforcing structure for: concrete paving. slabs comprisinglongitudinal'bars adapted in the 'lllitill'ilO lie in the lower portionof the side edges-of thecompleted slab, the ends of said bars beingb'entupwardly and thenex-tending longitudinallyto lie. in

the upper portion of the slab side'edges, other longitudinal barsextendlngzfromend to end of the-side edges at top and bottom,respectively, and transverse bars extending from sideto side at the endedges, and positioned at the top and bottom, respectii ely, stirrups--connecting the upper and lower transverse bars, said bent longitudinalbars being: connected to the upper and lower longitudinal. bars, andsaidtransverse and longitudinal bars being secured together to form acomplete, portable structure, transverse'bars supported; fromanchsecurcd to tion oft-heside-edge of the slab, th e elltlSOiii' saidbar being bent upwardly and extendingto the'ends oi the slab',-andadapted'to lie in the uppen portionof-the side edge thereof,.asecondlongitudinal bar extending:

between :the end portions of said lirst bar, to lie in the upperportionofthe slab edge, and meansior securing together the ends o'tsaiil bars;-

Signed at- Seattle, King County, lVasln ington, this 24th day of July1925.

GEORGE D. BURR;

